Zea mays or corn is an agronomically important crop in many countries of the world and is extremely commercially important in the United States. Corn is used for feed, for food and for industrial purposes.
Within the past 60 years, hybrid corn has become commercially dominant. As a result of the hybridization of corn, varieties with markedly improved yields, better stalks, better roots, markedly more uniform characteristics, and improved resistance to insect and disease pests have been developed.
Single cross hybrid corn is produced by using a homozygous inbred line as the parent. Homozygosity in an inbred line is achieved by repeated inbreeding and in general, by the sixth or seventh generation the inbred line is considered genetically pure. Unfortunately, a reduction in performance, yield and other plant characteristics, which are desirable, arises as a result of the inbreeding to achieve essentially genetic identity. As a result, these inbred corn lines are not grown as a commercial crop.
Although inbred lines which have been developed by various breeders in corn research are not grown as a commercial crop, they are extremely important because they are employed to produce first generation (F.sub.1) hybrids by the hybridization of, for example, two inbred lines as parents. As a result of the crossing of the two inbred lines, hybrid vigor or heterosis arises and the hybrid plants produced have markedly improved yields, better stalks, better roots, better uniformity and better insect and disease resistance. Further, as a result of self-pollination of F.sub.1 hybrid plants or cross-pollination of F.sub.1 hybrid plants, a second generation (F.sub.2) hybrid occurs due to the self-pollination or crosspollination of F.sub.1 hybrid plants. The F.sub.2 hybrid plant, and seed produced thereby, has characteristics which are less desirable than those of the F.sub.1 hybrid, for example, lower yields, and expression of undesirable genetic traits results. Due to this reduced performance, seed from F.sub.1 hybrids which produces less advantageous second generation F.sub.2 hybrids is not saved by farmers. Rather, new hybrid seed produced by crossing the originally selected inbred parents to produce the first generation hybrid (F.sub.1) seed is purchased from commercial seed companies by farmers each year for their planting.
The selection and production of inbred lines as parents which when crossed provide superior and improved F.sub.1 hybrids is a specialized and highly skilled area. Not only must a corn breeder select and develop superior inbred parental lines, but the corn breeder must also be able to produce and select hybrid combinations of these inbred lines which will produce desirable F.sub.1 hybrids which will be commercially successful.
As a result of the necessity to utilize inbred parental lines to produce commercially desirable and successful F.sub.1 corn hybrids, great emphasis is placed in corn breeding and hybridization to develop the necessary inbred parental lines. As a result, it is essential to select and develop new inbred lines which when used in hybridization will provide improvements in first generation hybrid corn characteristics in terms of increased yield, improved plant stability, improved resistance to disease and other insect pests, uniformity in appearance to thereby permit easy mechanical harvesting and maximization of harvesting efforts with a minimization of labor involved, etc.
As a result, a concerted effort exists in the production of commercially successful first generation corn hybrids to develop the necessary inbred lines as parental stock.